The invention pertains to a method and to an apparatus for detecting defects in rotational speed sensors.
It is known that the braking or forward propulsion of a motor vehicle can be influenced by interventions in the operation of the wheel brakes. This is usually done during braking processes to avoid excessive brake slip (antilock systems), in forward propulsion systems to avoid excessive drive slip (drive slip systems), and generally to stabilize the driving dynamics (driving dynamics control systems). It is essential to systems of this type to know when and to what extent the wheels are slipping (braking slip or drive slip). For this purpose, the rotational speeds or the rpm's of the wheels of the vehicle are usually detected by rotational speed sensors.
It is essential to the success of such systems that defects in the speed detection process be recognized, because defects of this type can lead under certain conditions to interventions in the brakes which have a critical effect on driving safety. One possible type of defect takes the form of, for example, a break in a connecting line; that is, the signal line which connects the rotational speed sensor to the control unit is interrupted. In addition, a signal can suddenly be lost during the operation of the vehicle even though the connecting line may still be intact; or it can occur that a speed signal is not being received at the brake control unit during the startup of the vehicle.
Rotational speed sensor defects of these kinds are usually "recognized" by means of corresponding plausibility tests at certain times before, during, or after vehicle operation. In plausibility tests of this type, the speed signals of certain individual wheels are compared with, for example, the speed signals of other wheels of the vehicle. If, for example, one of the speed signals deviates significantly from the other speed signals, then, under consideration of the operating status of the motor vehicle at the moment in question (e.g., spinning wheels), it is possible for the presence of a defect to be recognized. A wide variety of such plausibility tests is known according to the state of the art. Essential to the implementation of these tests is the determination of the operating points at which such tests are conducted.